Selective amplification of polynucleotides represents a major research goal of molecular biology, with particular importance in diagnostic and forensic applications, as well as for general manipulations of genetic materials and laboratory reagents.
The polymerase chain reaction (PCR) is a method by which a specific polynucleotide sequence can be amplified in vitro. PCR is an extremely powerful technique for amplifying specific polynucleotide sequences, including genomic DNA, single-stranded cDNA, and mRNA among others. As described in U.S. Pat. Nos. 4,683,202, 4,683,195, and 4,800,159 (which are incorporated herein by reference), PCR typically comprises treating separate complementary strands of a target nucleic acid with two oligonucleotide primers to form complementary primer extension products on both strands that act as templates for synthesizing copies of the desired nucleic acid sequences. By repeating the separation and synthesis steps in an automated system, essentially exponential duplication of the target sequences can be achieved.
A number of variations of the basic PCR methodology have been described. U.S. Pat. No. 5,066,584 discloses a method wherein single stranded DNA can be generated by the polymerase chain reaction using two oligonucleotide primers, one present in a limiting concentration. U.S. Pat. No. 5,340,728 discloses an improved method for performing a nested polymerase chain reaction (PCR) amplification of a targeted piece of DNA, wherein by controlling the annealing times and concentration of both the outer and the inner set of primers according to the method disclosed, highly specific and efficient amplification of a targeted piece of DNA can be achieved without depletion or removal of the outer primers from the reaction mixture vessel. U.S. Pat. No. 5,286,632 discloses recombination PCR (RPCR) wherein PCR is used with at least two primer species to add double-stranded homologous ends to DNA such that the homologous ends undergo in vivo recombination following transfection of host cells.
Horton et al. (1989) Gene 77: 61, discloses a method for making chimeric genes using PCR to generate overlapping homologous regions. In the Horton method, fragments of different genes that are to form the chimeric gene are generated in separate polymerase chain reactions. The primers used in these separate reactions are designed so that the ends of the different products of the separate reactions contain complementary sequences. When these separately produced PCR products are mixed, denatured and reannealed, the strands having matching sequences at their 3'-ends overlap and act as primers for each other. Extension of this overlap by DNA polymerase produces a molecule in which the original sequences are spliced together to form the chimeric gene.
Silver and Keerikatte (1989) J. Virol. 63: 1924 describe another variation of the standard PCR approach (which requires oligonucleotide primers complementary to both ends of the segment to be amplified) to allow amplification of DNA flanked on only one side by a region of known DNA sequence. This technique requires the presence of a known restriction site within the known DNA sequence and a similar site within the unknown flanking DNA sequence which is to be amplified. After restriction and recircularization, the recircularized fragment is restricted at an unique site between the two primers and the resulting linearized fragment is used as a template for PCR amplification.
Triglia et al. (1988) Nucl.Acids Res. 16: 8186, describe an approach which requires the inversion of the sequence of interest by circularization and re-opening at a site distinct from the one of interest, and is called "inverted PCR." A fragment is first created in which two unknown sequences flank on either side a region of known DNA sequence. The fragment is then circularized and cleaved with an unique restriction endonuclease which only cuts within the known DNA sequence creating a new fragment containing all of the DNA of the original fragment but which is then inverted with regions of known sequence flanking the region of unknown sequence. This fragment is then utilized as a PCR substrate to amplify the unknown sequence.
Vallette et al. (1989) Nucl.Acids Res. 17: 723 disclose using PCR in a specific approach which involves using a supercoiled plasmid DNA as a template for PCR and a primer bearing a mutated sequence which is incorporated into the amplified product. Using this method, DNA sequences may be inserted only at the 5'-end of the DNA molecule which one wishes to alter. Mole et al. (1989) Nucl.Acids Res. 17: 3319, used PCR to create deletions within existing expression plasmids. However, PCR was performed around the entire plasmid (containing the fragment to be deleted) from primers whose 5'-ends defined the region to be deleted. Self-ligation of the PCR product recircularized the plasmid.
U.S. Pat. No. 5,279,952 discloses a method for using PCR to generate mutations (e.g., deletions) and chimeric genes by forming head-to-tail concatemers of a known starting sequence and employing at least two PCR primers to amplify a DNA segment which is altered as compared to the known starting sequence.
Jones and Howard (1990) BioTechniques 8: 178, report a site-specific mutagenesis method using PCR, termed recombinant circle PCR (RCPCR). In RCPCR, separate PCR amplifications (typically two) of a known polynucleotide generate products that, when combined, denatured, and annealed, form double-stranded DNA with discrete, cohesive single-stranded ends designed so that they may anneal and form circles of DNA.
Oliner et al. (1993) Nucl. Acids. Res. 21: 5192, report a method for engineering PCR products to contain terminal sequences identical to sequences at the two ends of a linearized vector such that co-transfection of the PCR product and linearized vector into a recombination-competent host cell results in formation of a covalently linked vector containing the PCR product, thus avoiding the need for in vitro ligation.
In spite of such recent advances, including PCR and its various modifications noted above, there exists a need for improved methods of identifying and cloning polynucleotides, for accurate in vitro amplification of selected polynucleotides, and for facile assembly of polynucleotides from a mixture of component oligonucleotides or polynucleotides without necessitating the use of DNA ligase. In particular, there is a need for a PCR amplification method which can be performed with (1) only a single primer species, or (2) with multiple overlapping polynucleotide fragments (or oligonucleotides) in the absence of a conventional PCR primer, and which can result in formation of an amplified product which can be a concatemer and/or which can be a covalently-closed circle. The present invention fulfills these and other needs.
The references discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the inventors are not entitled to antedate such disclosure by virtue of prior invention. All publications cited are incorporated herein by reference.